1. Field of the Invention
The present invention relates to flux mapping systems for nuclear reactors and, more particularly, to verifying insertion of detectors into a desired thimble in a flux mapping system.
2. Description of the Related Art
Nuclear reactors include incore and excore detectors, i.e., detectors which are positioned inside the nuclear reactor core and detectors which are outside the core but inside the containment. Flux detectors, for example, may be used both inside the core and outside the core. In both cases, the flux detectors measure neutron flux emanating from the fuel in the core. The incore flux detectors are inserted into thimbles which pass through the core. The incore flux detectors are then drawn out of the core at a predetermined rate to produce a flux map along each thimble.
A pressurized water nuclear reactor typically has approximately sixty thimbles, but may have only four detectors. Multiple-path selectors are used to route the detectors into the thimbles. All or a portion, e.g., a quadrant, of the core may be mapped at a given time. Typically, all, e.g., four detectors are inserted into different thimbles simultaneously so that the map can be produced as quickly as possible while conditions in the core undergo little change.
To produce an accurate core map, it is necessary to determine into which thimbles the detectors are inserted. This is termed path insertion verification. It is not uncommon for mechanical malfunctions of path insertion verification switches to reduce the availability of flux mapping systems. Flux maps are typically produced once a month to determine the decay rate of fuel in the reactor. The path verification switches in a conventional system are located in containment; thus, repair of a malfunctioning switch exposes maintenance workers to undesirable levels of radioactivity. In addition, flux mapping system availability has been reduced while waiting for a plant shutdown, e.g., for refueling, to repair the path verification switches. Also, low reliability of the path insertion switches has reduced availability of flux mapping systems by requiring all detectors to be fully withdrawn prior to starting a flux map.